The present invention is directed to avionic systems for measuring height above ground level for terrain avoidance during flight and landing in general, and more particularly, to a system for profiling objects on the terrain forward and below the aircraft utilizing a cross-track laser altimeter to avoid collision therewith during flight and landing.
Today's military and commercial aircraft desire more precise measurements of aircraft position and position information. Ground based RADAR systems and Global Positioning Systems (GPS) allow for precise positioning of an aircraft in latitude and longitude desirable for air traffic control, aircraft separation, and navigation. However, precise measurements of altitude are often difficult to achieve with such systems, especially for applications in which precise placement above ground is needed. New levels of precision for altitude measurements, on the order of +/−6 inches (15 cm), for example, are often required for flight profiles ranging from hover, to nap of the earth (NOE) flight, and autonomous landing.
Although a GPS has the ability to determine altitude of the aircraft, without a differential GPS receiver, precision above ground level (AGL) information is not possible due to the approximations in the GPS altitude estimation and the particular geodetic datum used. Systems have been developed to augment this limitation in GPS AGL accuracy by referencing GPS latitude and longitude to a terrain elevation map stored digitally in the avionics. With GPS alone, AGL accuracy can be on the order of 20 feet or more, but adding the digital terrain map reference allows for compensation to a suitable AGL measurement for navigation. However, such an augmented system does not provide for other information, such as the presence of trees and other ground objects, for example, which is critical for avoiding collisions during hover and NOE flight conditions.
Commercial and military aircraft often employ a barometric pressure altimeter for a common altitude reference measurement. With known airport ground level elevations relative to sea level, navigation of an aircraft during landing can be easily accomplished using barometric pressure altitude readings to establish height above the ground level of the airport. With this system, AGL accuracy is commonly on the order of units of feet. However, this technique does not account for ground features such as buildings, power lines, and other ground objects resulting in separation differences between the object, ground, and aircraft.
In each case, these AGL measurements are often insufficient for manned or unmanned aircraft flight profiles when exact AGL distancing is required. Further, digital terrain and object mapping data used in navigation may not always reflect changes in terrain, buildings, or other ground obstacles.
For military applications, it is sometimes necessary for aircraft to minimize the height above ground during flight to avoid detection. Thus, military aircraft, especially unmanned air vehicles (UAVs), that fly near the terrain for mission execution or autonomous landing require sensory data to not only accurately measure height above ground, but also height above ground objects as well. To address this problem in cruise missiles and other unmanned aircraft, radar altimeters were developed and employed to operate in radio frequency (RF) bands in frequency modulated (FM) continuous wave (CW) and pulsed, time of flight systems. Using a time of flight technique, radar altimeters transmit a radar pulse towards the ground and receive a reflection of the transmitted pulse from the ground. Range or distance R from the aircraft to the ground feature is determined from a formula R=c*T/2, where c is the speed of light, and T the measured round trip time of flight of the radar pulse.
Although radar altimeters are widely used to measure AGL height for pilot reference, they often do no provide the necessary spatial resolution required by the aircraft guidance and control systems for autonomous landings or control. With these commonly available systems today, AGL measurement precision can vary from 1-3%, which is primarily a result of the radiated pulse beam width, often having a solid angle as large as units of degrees. Also, as the pulsed beams strike the uneven terrain surface, different AGL heights are measured, which affect the precision of the overall aircraft AGL height measurement, the accuracy of which being a function of AGL height. In these systems, careful attention to installation of the radar altimeter to the aircraft is also needed to ensure isolation of the radiated field and so that multi-path secondary reflections do not confuse the receiver electronics. This multi-path interaction between the aircraft and the radar altimeter combined with the requirements for field isolation result in a limited number of locations on the aircraft where the device may be installed for practical applications of use.
Accordingly, a system is needed to measure height of the aircraft above ground including ground objects with the desired precision for autonomous control and landings, especially for UAVs. The present invention overcomes the drawbacks of the present systems and provides for the profiling of ground objects for use in determining the AGL height precision desired utilizing a cross-track laser altimeter.